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A Slingshot From Earth To Asteroid Bennu

September 22, 2017 – NASA’s asteroid sample return spacecraft successfully used Earth’s gravity on Friday to slingshot itself on a path toward asteroid Bennu, for a rendezvous next August.

At 10:52 a.m. MDT on September 22, the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer) spacecraft came within 10,711 miles (17,237 km) of Antarctica, just south of Cape Horn, Chile, before following a route north over the Pacific Ocean.

OSIRIS-REx launched from Cape Canaveral Air Force Station in Florida on September 8, 2016, on a United Launch Alliance (ULA) Atlas V 411 rocket. Although the rocket provided the spacecraft with the all the momentum required to propel it forward to Bennu, OSIRIS-REx needed an extra boost from the Earth’s gravity to change its orbital plane.

“This is not about gaining speed,” said Erin Morton, communications lead for the OSIRIS-REx mission. “We have all the speed we need to catch up with Bennu. But to get onto the asteroid’s orbit, OSIRIS-REx has to boost itself slightly above the plane on which Earth travels around the solar system. Placing it onto that trajectory at launch would have required excessive amounts of rocket fuel, so we’re using Earth’s gravity instead.”

The team has already made multiple adjustments to the spacecraft’s path since launch on September 8, 2016. The largest was a deep space maneuver on December 28, 2016, that changed the speed and path of the spacecraft to target Earth for the flyby. There have also been three trajectory correction maneuvers – one on October 7, 2016, one on January 18, 2017, and another on August 23, 2017 (30 days before the gravity assist) – that further refined the spacecraft’s trajectory in preparation for the flyby.

The navigation team comprises employees from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and KinetX Aerospace. KinetX Aerospace navigation team members plan and carry out all OSIRIS-REx maneuvers with the Lockheed Martin spacecraft operations team at the Lockheed Martin Waterton Campus in Littleton, Colorado.

For the Earth Gravity Assist, the navigation team calculated the required amount of change in the spacecraft’s course and speed. This information was then translated by the operations team into commands that were uploaded to the spacecraft and executed by firing the spacecraft’s rocket engines.

Bennu’s orbit around the Sun is tilted six degrees from Earth’s orbit, and today’s maneuver changed the spacecraft’s direction to put it on the path toward Bennu. As a result of the flyby, the velocity change to the spacecraft was 8,451 miles per hour (3.778 kilometers per second).

Image Credit: University of Arizona

The mission team is also using OSIRIS-REx’s Earth flyby as an opportunity to test and calibrate the spacecraft’s instrument suite. Approximately four hours after the point of closest approach, and on three subsequent days over the next two weeks, the spacecraft’s instruments will be turned on to scan Earth and the Moon. These data will be used to calibrate the spacecraft’s science instruments in preparation for OSIRIS-REx’s arrival at Bennu in late 2018.

“The opportunity to collect science data over the next two weeks provides the OSIRIS-REx mission team with an excellent opportunity to practice for operations at Bennu,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “During the Earth flyby, the science and operations teams are co-located, performing daily activities together as they will during the asteroid encounter.”

The OSIRIS-REx spacecraft is currently on a seven-year journey to rendezvous with, study, and return a sample of Bennu to Earth. This sample of a primitive asteroid will help scientists understand the formation of our solar system more than 4.5 billion years ago.

NASA’s Goddard Space Flight Center provides overall mission management, systems engineering and the safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona, Tucson, is the principal investigator, and the University of Arizona also leads the science team and the mission’s science observation planning and data processing. Lockheed Martin Space Systems in Denver built the spacecraft and is providing flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA’s New Frontiers Program. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the agency’s New Frontiers Program for the Science Mission Directorate in Washington.